U.S. patent application number 12/770791 was filed with the patent office on 2011-11-03 for decomposition with multiple exposures in a process window based opc flow using tolerance bands.
This patent application is currently assigned to INTERNATIONAL BUSINESS MACHINES CORPORATION. Invention is credited to Ioana C. Graur, Geng Han, Scott M. Mansfield.
Application Number | 20110271238 12/770791 |
Document ID | / |
Family ID | 44859334 |
Filed Date | 2011-11-03 |
United States Patent
Application |
20110271238 |
Kind Code |
A1 |
Mansfield; Scott M. ; et
al. |
November 3, 2011 |
DECOMPOSITION WITH MULTIPLE EXPOSURES IN A PROCESS WINDOW BASED OPC
FLOW USING TOLERANCE BANDS
Abstract
Setting final dimensions while protecting against the
possibility of merging shapes is provided by performing a
decomposition of tolerance bands onto a plurality of masks for use
in a multi-exposure process. This allows the maximum process
latitude between open and short failure mechanisms, while also
providing a mechanism to enforce strict CD tolerances in critical
regions of a circuit. The decomposition enables co-optimizing
various types of shapes placed onto each mask along with the source
used to print each mask. Once the tolerance bands are decomposed
onto the two or more masks, standard tolerance-band-based data
preparation methodologies can be employed to create the final mask
shapes.
Inventors: |
Mansfield; Scott M.;
(Hopewell Jct., NY) ; Han; Geng; (Fishkill,
NY) ; Graur; Ioana C.; (Poughkeepsie, NY) |
Assignee: |
INTERNATIONAL BUSINESS MACHINES
CORPORATION
ARMONK
NY
|
Family ID: |
44859334 |
Appl. No.: |
12/770791 |
Filed: |
April 30, 2010 |
Current U.S.
Class: |
716/55 ;
716/50 |
Current CPC
Class: |
G03F 1/36 20130101; G03F
1/70 20130101; G03F 7/203 20130101; G03F 7/70466 20130101; G06F
30/00 20200101 |
Class at
Publication: |
716/55 ;
716/50 |
International
Class: |
G06F 17/50 20060101
G06F017/50 |
Claims
1. A method of creating multiple mask designs used in a multiple
exposure lithographic patterning process, each of said mask design
including a plurality of shapes, the mask design being used for
printing a design layout on a semiconductor wafer, the method
comprising: a) using a computer, creating a first set of tolerance
bands (TBs) for each edge of each shape in the design layout, said
first set of TBs defining the tolerance of edge locations of the
printed edges; b) decomposing said first set of the TBs of the
design layout to create a second set of TBs for each mask; and c)
applying the second set of TBs to create mask shapes which when
printed in a multiple exposure lithographic patterning process
result in wafer shapes having edges falling within the first set of
TBs of the design layout.
2. The method as claimed in claim 1, wherein the first set of TBs
includes an original set of TBs for the design layout.
3. The method as claimed in claim 1, wherein said creating said
mask design is achieved by performing a correction for
non-linearity of process.
4. The method as claimed in claim 3, further comprising correcting
process non-linearities, said corrections including OPC, etch
corrections, and mask manufacturing corrections.
5. The method as claimed in claim 1, further comprising mapping
critical components wherein all edge segments of the first set of
TBs, include both inner and outer band segments mapped to exactly
one edge of the design layout.
6. The method as claimed in claim 1 further comprising mapping
tolerance band edges onto design edges, forming a mask design based
on decomposition and on original design edges.
7. The method as claimed in claim 6, wherein the decomposed design
edges and corresponding tolerance band edges are copied onto the
same masks as the design edges.
8. The method as claimed in claim 1, further comprising for each
break in the design shape created during decomposition, creating a
new inner tolerance band edge at corresponding vertices, with inner
tolerance band edges are created on all the masks affected by said
break in the design shape.
9. The method as claimed in claim 8, wherein for each mask affected
by the break in the design shape, all other masks affected are
assessed by the break, and wherein the new inner tolerance band
edge are projected in direction of the interior of the inner
tolerance until abutting at an opposing edge.
10. The method as claimed in claim 1, further comprising
decomposing the tolerance bands by mapping each line segment of the
tolerance band to a line segment on the original mask design.
11. The method as claimed in claim 6, further comprising copying
associated tolerance band edge segments onto a mask design instead
of or in addition to the original design edges.
12. The method as claimed in claim 6, further comprising creating
rectangles based on the projections and keeping an overlapping
region of all of the rectangles.
13. The method of claim 1, further comprising: d) for each break in
the design shape created when decomposing, generating a new inner
tolerance band edge at corresponding vertices, the inner tolerance
band edges being created on all the masks are affected by a break
in the design shape; e) for each of the masks affected by the break
in the design shape, determining any other mask affected by the
break and projecting the new inner tolerance band edge in a
direction of the interior of the inner tolerance until it reaches
an opposite edge; f) creating rectangles based on the projections
and keeping overlapping regions of all of the rectangles; g) moving
orthogonal edges of the new rectangle outward until they reach an
outer tolerance band of one of the other affected masks; and h)
moving cut vertices of the outer tolerance band being fixed until
they reach a new shape.
14. A program storage device readable by a machine, tangibly
embodying a program of instructions executable by the machine to
perform method steps of creating multiple mask designs used in a
multiple exposure lithographic patterning process, each of said
mask design including a plurality number shapes, the mask design
being used for printing a design layout on a semiconductor wafer,
the method steps comprising: a) using a computer, creating a first
set of tolerance bands (TBs) for each edge of each shape in the
design layout, said first set of TBs defining the tolerance of the
edge locations of the printed edges; b) decomposing the first set
of TBs of the design layout to create a second set of TBs for each
mask; and c) applying the second TBs to create mask shapes which
when printed in the multiple exposure lithographic patterning
process result in wafer shapes having edges falling within the
first TB of the design layout.
Description
FIELD OF THE INVENTION
[0001] The present invention relates generally to the field of
Design Automation, and more particularly, to a method and data
structure that enables decoupling resolution enhancement techniques
to allow optimum printing of patterns.
BACKGROUND AND RELATED ART
[0002] With the need to print patterns at dimensions close to and
in some cases beyond the physical limits of optical lithographic
techniques, various techniques have been developed to enhance the
resolution of optical exposure tools. One of the more extreme
classes of these Resolution Enhancement Techniques (RET) is the use
of multiple separate exposures to print a single pattern. Through
the use of multiple exposures, the physical resolution limitations
of an optical system can be overcome. However, multiple exposures
alone cannot ensure adequate pattern fidelity for the multitude of
layout geometries that exist in a standard semi-conductor design,
and typically several RET techniques must be combined to achieve
required tolerances and process latitudes. Unfortunately, RET
techniques are often not independent of each other and the order in
which they are applied may have a significant impact on the final
patterning quality. As a result, it becomes necessary to find a
methodology and data structure that enables decoupling of the RET
techniques and allows for optimized solutions to be found.
[0003] Conventional single exposure optical lithography is limited
in resolution to a minimum feature periodicity of k1 * .lamda./NA,
where .lamda. is the wavelength of the exposure radiation, NA is
the numerical aperture of the lens and k1 is a value that
represents the difficulty of the imaging. K1 for a single exposure
process is physically limited to a minimum value of 0.25, but for
practical purposes in a manufacturing process with real process
variation, k1 factors below 0.75 can cause significant challenges
to the patterning process. In such "low k1" imaging scenarios, many
techniques are used to improve pattern fidelity. Resolution
Enhancement Techniques (RET) include off-axis illumination, assist
features and Optical Proximity Correction (OPC). Off-axis
illumination typically improves the printing of some types of
patterns at the expense of other pattern types. In addition,
off-axis illumination may lead to strong nonlinearities in the
patterning process, so that features that are the same size on the
photomask may print at different sizes on the wafer. These types of
nonlinearities are often corrected using OPC, but in the extreme
cases, the nonlinearities may result in some pattern sizes that
simply cannot be achieved on the wafer. In those cases, a design
retargeting step may be required, where forbidden patterns are made
larger in size in the design layout. Assist features are often used
to make all patterns on the photomask look more uniform in pattern
density. If the assist features are smaller than the design
features, then they will not transfer onto the wafer during the
printing process. However, by making the photomask patterns more
uniform, the patterning process can be better optimized. For a
standard single exposure patterning process, the photomask design
is typically created by first optimizing the illuminator, then
applying retargeting, adding assist features and finally
manipulating the mask sizes with OPC to ensure all patterns print
at their target dimensions. Often, the entire process is referred
to as OPC and that will be the case for the remainder of the
application.
[0004] With design rules for advanced semiconductor processes now
pushing required k1 factors down to 0.25 or smaller, single
exposure patterning processes are no longer sufficient. To deal
with this, one RET technique that is used is breaking up
(decomposing) a design level into multiple mask layers which can be
used to create a final wafer pattern by exposing each mask pattern
in succession, with each mask either creating some portion of the
final pattern or erasing unwanted features from a previous
exposure, as shown, e.g., in FIGS. 1(a) through 1(c) wherein the
design layout is illustrated consisting of shapes 101-104. These
shapes can be decomposed into primarily vertical shapes 105-107 and
horizontal shape 108. Mask A (117) can be created by applying
industry standard process corrections, such as OPC, resulting in
Mask A shapes 109-111. Mask B (118) shape 112 can be created in a
like manner. Mask A and Mask B can be manufactured and used to
print patterns on a semiconductor wafer using an industry standard
multiple-exposure patterning processes. The result of this process
is shown by wafer shapes 113-116 that are superimposed on their
initial design shapes (101-104), as shown in FIG. 1d. The
aforementioned illustrative example can be recognized having many
variations on the process, some of which will be discussed
hereinafter.
[0005] Moreover, still referring to FIG. 1c, the decomposed mask
shapes are depicted separated to clearly illustrate the two masks
117 and 118. In subsequent figures, the separation will be assumed
and the mask shapes for multiple masks will be drawn as if they
were physically located next to each other, the mask distinction
being shown by shape shading.
[0006] The RET is often divided into two broad categories: double
exposure (DE) and double patterning (DP). In DE, multiple exposures
are made into the same photoresist layer and that layer is then
developed and processed to create the final pattern. In DP, a
photoresist layer is only exposed with a single mask and then
processed to create an intermediate transfer layer prior to another
photoresist layer being added, exposed and processed. It is
understood that these general categories can be extended beyond two
masks to include multiple exposures (ME) and multiple patterning
(MP); however, in the present application, only DE and DP will be
considered. Likewise, many variations of DE and DP exist in the
literature and it is understood that many of these variations will
also benefit from the methodologies outlined in the application.
For clarity, in the subsequent discussion, no distinction will be
made between DE and DP, and in all cases, these will just be
considered double exposure DE.
[0007] To enable DE patterning solutions, the shapes on a
semiconductor design level no longer permit having a one-to-one
mapping with shapes on a photomask that can be used to print that
level on the wafer. Instead, a complex mapping of the design shapes
to mask shapes must occur through a process known as decomposition.
For instance, a typical DE decomposition methodology involves
moving all horizontal shapes onto one mask and all vertical shapes
onto the second mask. The decomposition is appropriate for a
patterning process using two highly asymmetric illuminators, such
as dipoles, often referred to as double-dipole-lithography (DDL).
Likewise, one DP approach attempts to double the spatial frequency
of printed gratings by printing every other line with one mask and
the intermediate lines on another. Often referred to as
pitch-splitting, this technique allows optical lithography to be
used beyond the k1=0.25 physical limit. Accordingly, it is apparent
that the nature of the decomposition process is highly dependent on
the patterning process that will be used, including various RETs
that may be used in that process.
[0008] In addition to multiple exposures, various other RETs are
required to create a final pattern. By way of example, the
illumination used in each of the exposures can be highly tuned to
print a particular type of pattern. As in the case of DDL, where
two dipoles are used to print patterns of different orientations,
an illuminator optimization can be used in conjunction with
decomposition to create highly optimized combinations of
illumination and mask shapes. Automated source-mask-optimization
(SMO) algorithms have been developed to achieve the highest degree
of optimization. Likewise, other mask optimization techniques, such
as sub-resolution assist features and phase shifting masks, can be
used to allow optimum printing of desired patterns. For low k1
lithography, optical proximity correction (OPC) is required to
pre-distort mask shapes in a manner that compensates for
nonlinearities in the printing process. Typically, combinations of
all of these RET are needed to achieve a robust patterning
solution.
[0009] Despite all of these techniques, the lithography process
that is exerted to its limits will still not allow all patterns to
be faithfully reproduced on the wafer in the presence of normal
manufacturing process variations. As a result, some design target
shapes must be modified to align with process capabilities. The
"retargeting" process often involves making isolated patterns
larger, making patterns with small areas larger and expanding
regions where patterns transition from one periodicity to another
(i.e., fan-out regions). In a conventional single exposure process,
retargeting can often be done based on rules applied to the target
geometries. The fact that all the target geometries will be printed
by the same exposure make it relatively easy to predict which
patterns will have printing problems and will need to be expanded,
even prior to the final mask shapes being determined, as shown in
FIG. 2. Herein, design shapes 101-104 are expanded or retargeted,
to create larger target shapes 205-208. Still in FIG. 2, a single
exposure process is used, so shapes 205-208 are used as target
shapes for OPC and mask shapes 209-212 are created. Printing the
mask onto a wafer using a single exposure patterning process
results in printed shapes 213-216 which are shown superimposed on
the original target shape 101-14 in FIG. 2d. The printed shapes are
bigger than the original targets, but may suffer from regions
having a high risk of shorting, 217, or an overall high sensitivity
to process variations due to the nature of the single exposure
process.
[0010] Applying a retargeting flow similar to the one for use in a
decomposed process may lead to significant patterning problems and
ultimately yield loss. The reason for that can be seen with
reference to FIG. 3, where similar to FIG. 2, a retargeting step is
used to expand all isolated edges in the original target design,
i.e., 205-208. The retargeted shapes are then decomposed based on a
DDL decomposition algorithm to create target shapes for two mask
designs, with 309-311 representing mask target shapes for Mask A
and 312 being a mask target shape for Mask B. Standard RET and OPC
data preparation techniques are then used to create mask designs,
consisting of OPC'ed shapes 313-315 for Mask A and OPC'ed shape 316
for Mask B. Printing the mask shapes with a double exposure
patterning process results in wafer contours 317-320. Since the
retargeting is done on the pre-decomposed layout, isolated regions
on each mask cannot be properly found and printing problems can
arise. For example, region 321 will be printed with a relatively
isolated mask shape, but will not have any retargeting done to
boost the process window. So, the impact of this becomes a risk of
the pattern pinching off.
[0011] Likewise, FIGS. 4 and 5 show similar problems found in a
real design layout. In FIG. 4, short horizontal segment 401 does
not receive any retargeting, but is printed as a relatively
isolated segment in a double exposure patterning process. The
resulting contour, based on simulating the printing process, shows
a tendency to pinch at region 402. Similar problems can be seen in
FIG. 5 where vertical segments 501-504 have many horizontal
segments in close proximity of one another. This results in no
retargeting of the design and subsequent printing problems in the
double exposure process, leading to regions of failure risk at
505-508.
[0012] The reason for the aforementioned failures lies in the fact
that when a decomposition technique is used, the process latitude
to print a particular target feature is not known at least until
the decomposition step is complete and, more accurately, once the
full RET/OPC steps are complete. So, it may seem reasonable to move
the retargeting step after the decomposition step, as shown in FIG.
6, where decomposed mask target shapes 105-108 are retargeted
according to their decomposed environments to create retargeted
target shapes 609-611 for Mask A and 612 for Mask B. In this case,
the local environment for each target pattern is known at the time
of retargeting, and the information can be used to accurately
predict which shapes will have a weak process margin and will need
to be retargeted. The retargeted target shapes can then be used to
create mask shapes 613-615 for Mask A, and 616 for Mask B. However,
this approach also has some drawbacks. If each decomposed mask
design is retargeted independently, then there will be locations
where target shapes on both masks will be expanded into the same
region. In this case, when the final pattern (617-620) is created
on the wafer, retargeted patterns will have a risk of bridging or
merging, together causing failure of the circuitry, for instance at
location 621.
[0013] Since the subsequent RET and OPC (optical proximity
correction) data processing that is carried out beyond the
retargeting and decomposition steps is dependent upon having a
known target that is to be reproduced on the wafer, it is not
feasible to move the retargeting step any further into the data
preparation flow. However, one can attempt to approximate that by
exercising process window OPC (PWOPC), where the OPC is not forced
to converge to the exact target shape, but is given some leeway to
print the shape off-target at nominal process conditions. The goal
of the PWOPC algorithm is typically to print all edges within some
pre-defined tolerance under normal process variation. The tolerance
information can be specified in the form of a tolerance band or
target band drawn or generated around each edge.
[0014] Referring to FIG. 7, an example of the use of tolerance
bands for process window OPC is illustrated. Therein, a single
exposure technique is depicted and retargeted shapes 205-208 are
created based on the full environment for each exposure. In this
case, tolerance bands are created using one set of rules that
describe an inner and outer tolerance for the width of the line and
another set of rules that describe an inner and outer tolerance for
the length of the line, the result of which consisting in outer
tolerance shapes 713, 715, 717 and 719, and inner tolerance shapes
714, 716, 718 and 720. Process window based OPC techniques can be
used to create mask shapes 721-724 which result in printed wafer
shapes 725-728 when printed with a single-exposure process.
Although this technique may allow a printing process to keep edges
within their tolerance through a broader range of process variation
than the standard retargeted single-exposure process of FIG. 2, it
still suffers from the deficiency of being a single exposure
process and will not provide adequate process latitude under
aggressive design rules.
[0015] Referring to FIG. 8, a more advanced method of creating
tolerance bands for a single exposure process is shown, as
described, e.g., in U.S. Pat. No. 7,266,798. In the cited patent,
multilayer checks are used to create tolerance bands that better
match the important electrical tolerances of the circuitry. These
tolerance bands account for the full three-dimensional nature of
the circuits when printed on the wafer and are created by analyzing
each layer in the design in the context of layers above and below
them. The result is a larger variation in the size of the tolerance
bands as defined by outer tolerance shapes 813, 815, 817 and 819,
and inner tolerance shapes 814, 816, 818 and 820. Process window
based OPC applied to the tolerance bands results in mask shapes
821-824 and wafer patterns 825-828. Such a technique helps to
reduce the failure risk. However, the prior art does not anticipate
pattern decomposition and, therefore, can only allow tolerance band
generation once each of the mask target layers have been fully
defined through decomposition. No methodology prior to now has been
disclosed or anticipated on how to optimize tolerance bands for a
multiple exposure process.
SUMMARY OF EMBODIMENTS OF THE INVENTION
[0016] Accordingly, there is a need in the semiconductor industry
for a method for generating tolerance bands on a pre-decomposed
layout and for decomposing the tolerance bands, rather than target
shapes, for use in subsequent data preparation for each of the
individual masks. In doing so, retargeting allowances can be well
balanced between each mask design while also ensuring no risk of
pattern failure.
[0017] In one aspect of the invention, a method and a data
structure are provided that enable decoupling the RET techniques,
making it possible to achieve improved and potentially optimized
results.
[0018] In another aspect, the invention decomposes tolerance bands
onto two or more masks for use in a multi-exposure process. By
using tolerance bands to convey the available target edge leeway,
data preparation steps that occur after the decomposition acquire a
certain desirable flexibility in setting the final printed
dimensions while also protecting against the possibility of merging
shapes. This allows maximum process latitude between open and short
failure mechanisms, while also providing a mechanism to enforce
strict CD tolerances in critical regions of a circuit.
[0019] Tolerance bands are created for a single design level or
multiple design levels using industry standard methodologies. The
tolerance bands are then decomposed and separated onto tolerance
band levels for two or more masks. The decomposition methodology
enables co-optimizing various types of shapes placed onto each
mask, along with the source used to print each mask. Once the
tolerance bands are decomposed onto the two or more masks, standard
tolerance-band-based data preparation methodologies can be employed
to create the final mask shapes.
[0020] In still another aspect, the invention provides a method of
decomposing tolerance bands onto multiple masks for use in a
multiple exposure lithographic imaging, the method including:
generating tolerance bands and mapping all tolerance band edges
onto corresponding target edges; decomposing the target edges and
copying corresponding tolerance band edges onto the masks as target
edges; for each break in a target shape created during the
decomposition, generating a new inner tolerance band edge at
corresponding vertices, the inner tolerance band edges being
created on all the masks affected by a break in the target shape;
for each of the masks affected by the break in the target shape,
determining any other mask affected by the break and projecting the
new inner tolerance band edge in the direction of the interior of
the inner tolerance until it reaches an opposite edge; creating
rectangles based on the projections and keeping overlapping regions
of all of the rectangles; moving the orthogonal edges of the new
rectangle outward until they reach an outer tolerance band of one
of the other affected masks; and moving cut vertices of the outer
tolerance band being fixed until they reach a new shape.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] The novel features believed characteristic of the invention
are set forth in the appended claims. The invention itself,
however, as well as a preferred mode of use, further objects and
advantages thereof, will best be understood by reference to the
following detailed description of an illustrative embodiment when
read in conjunction with the accompanying drawings, where:
[0022] FIG. 1 is a schematic diagram of a prior art illustrative
example of decomposing a design level into multiple mask layers by
exposing each mask pattern in succession, with each mask creating a
portion of the final pattern and erasing unwanted features from a
previous exposure.
[0023] FIG. 2 shows prior art illustrations of a retargeting
process making isolated patterns larger, making patterns with small
areas larger and expanding regions where patterns transition from
one periodicity to another.
[0024] FIGS. 3, 4 and 5 show prior art illustrations of a
decomposition process based on a retargeted designs, including
simulations of the printed wafer contours showing the process
results in patterns with a pinching risk.
[0025] FIG. 6 illustrates a prior art process where a layout is
first decomposed and then retargeted, including example post-OPC
mask shapes and a simulation of the printed wafer contours.
[0026] FIG. 7 shows a diagram illustrating the use of tolerance
bands for a process window OPC in which a single exposure and
retargeted shapes are created based on the full environment for the
exposure.
[0027] FIG. 8 shows a diagram illustrating a second use of
tolerance bands for a process window OPC in which a single exposure
is shown and retargeted shapes are created based on the full
environment for the exposure.
[0028] FIG. 9 is an illustrative example showing decomposing the
tolerance bands followed by ensuring that the tolerance bands
define whole, closed shapes, wherein each line segment of the
tolerance band is mapped to a line segment on the original design
target.
[0029] FIG. 10 shows an illustrative example of a design clip
decomposed, wherein tolerance band edges are mapped onto design
target edges and wherein mask designs based on a decomposition
algorithm instead of or in addition to the original design target
edges, according to an embodiment of the invention.
[0030] FIG. 11 illustrates examples of a T-shaped target pattern
and tolerance band decomposed into horizontal and vertical
components.
[0031] FIG. 12 shows an illustrative example of corresponding outer
tolerance band edges created for a first mask once the new inner
tolerance band segments are created, and where the outer band on
one mask are found after all the other masks have been
considered.
[0032] FIG. 13 shows an illustrative example of corresponding outer
tolerance band edges created for a second mask once the new inner
tolerance band segments are created, and where the outer band on
one mask are found, all the other masks having been considered.
[0033] FIG. 14 depicts the patterns resulting when the masks are
used to print patterns in photoresist. FIG. 14(a) shows contours
respectively printed by Mask A and by Mask B, wherein an overlap
between the shapes that are printed by each mask is created to
ensure continuity of the desired T-shape. FIG. 14(b) shows a second
set of contours resulting from the two masks printing on the small
side of their expected tolerance, wherein the smaller overlap
region potentially may raise yield concerns.
[0034] FIG. 15 shows a method aimed for improving worst-case
overlap of both exposures. Process window OPC algorithms may work
better if there is a better match between PVB and width and the
tolerance band width.
[0035] FIGS. 16 and 17 illustrate further improvements limiting the
extent of outer tolerance bands. Some overlap region potentially
may raise yield concerns.
[0036] FIG. 18 illustrates improved contours, and more
particularly, an improved worst case overlap.
[0037] FIG. 19 is a flow chart illustrating a preferred embodiment
of the present invention.
PREFERRED EMBODIMENTS OF THE INVENTION
[0038] The present invention will now be described in greater
detail by referring to the following discussion with reference to
the drawings that accompany the present application. It is observed
that the drawings are provided for illustrative purposes and thus,
they are not drawn to scale.
[0039] In one embodiment, the decomposed tolerance bands may be
created by first decomposing the tolerance bands, followed by a
clean-up step to ensure that the tolerance bands define whole
closed shapes. To decompose the tolerance bands, each line segment
of the tolerance band is mapped into a line segment on the original
design target, as shown in FIG. 9. Depending on how the tolerance
bands are created, mapping may already exist and show a one-to-one
relationship. If the mapping does not exist, then it may be
created, although it is not a critical aspect of the invention that
a one-to-one mapping exists. The critical component is that all the
edge segments of the tolerance band including both the inner and
outer band segments be mapped to exactly one edge on the original
design target.
[0040] By way of example, and still referring to FIG. 9, two
original design target shapes 901 and 902 are depicted. Shape 901
consists of edges 903, 906, 909 and 912. Standard methods can be
used to create outer tolerance band edges 904, 907, 910 and 913.
The inner tolerance band edges 905, 908, 911 and 914 are then
formed, in which case, tolerance band edges 904 and 905 are mapped
to target edge 903; 907 and 908 to 906; 910 and 911 to 909; and
913, 914 to 912. Design target shape 902 has similar mapping of
tolerance band edges, although in this case, an outer tolerance
band has had a jog introduced, as illustrated by the three edge
segments 916, 918 and 919. Accordingly, the outer tolerance band
edges 916, 918 and 919 along with the inner tolerance band edge 915
are mapped onto target edge 915. The remaining tolerance band edges
can be advantageously mapped in a similar manner, with 921 and 922
mapped onto 920, and the like.
[0041] Upon having the tolerance band edges mapped onto the design
target edges, the decomposition can proceed as normal, wherein edge
segments of the design target are copied to one of two or more mask
designs based on a decomposition algorithm. However, in this
instance, associated tolerance band edge segments may preferably be
copied onto the mask designs instead of or in addition to the
original design target edges.
[0042] Referring to FIG. 10, an example of a larger design clip
decomposed is illustrated using a methodology, wherein the
tolerance bands of FIG. 8, based on outer and inner tolerance
shapes 813-820 are decomposed into outer tolerance shapes for Mask
A 1013, 1015 and 1017 and inner tolerance shapes for Mask A 1014,
1016, 1018. The outer tolerance shape for Mask B 1019 and inner
tolerance shape for Mask B 1020 are decomposed in a similar manner.
Mask shapes 1021-1023 of Mask A and 1024 of Mask B are then
created, preferably using a process window based OPC for each of
the two exposures. The resulting wafer patterns after the
double-exposure printing process 1025-1028 show a significantly
improved process latitude due to the combination of tolerance bands
and double exposure.
[0043] During the decomposition process, the algorithm may
advantageously introduce breaks in the tolerance bands, so that the
bands inhibit the closed target pattern to be created. This occurs
in regions where two or more mask layers are required to create a
continuous wafer pattern. By way of example, in a T-shaped pattern,
the algorithm may break the closed T-shaped polygon into two
separate polygons by placing the horizontal and vertical legs on
different masks.
[0044] Referring to FIG. 11, T-shaped target pattern 1101 is
decomposed into horizontal component 1104 and vertical component
1107. Likewise, outer tolerance band 1102 is decomposed into a
corresponding horizontal component 1105 and vertical component
1108, while inner tolerance band 1103 is decomposed into 1106 and
1109. The decomposition of the T-pattern into horizontal and
vertical components causes a break in target shape at 1110,
resulting in shapes 1104 and 1107 being open shapes. Likewise,
outer tolerance band shapes 1105 and 1108 have breaks at 1111, and
inner tolerance band shapes 1106 and 1109 have breaks at 1112.
Neither the inner tolerance band nor the outer tolerance band
shapes are closed shapes. Combining both masks is required to
create a final continuous T-pattern on the wafer. In such instance,
a clean-up step is required to introduce new edge segments in
overlapping regions. In the case of a T-pattern, additional edge
segments are required on both legs, i.e., to fill in the bottom
horizontal edge of the horizontal leg and to create the top line
end of the vertical leg.
[0045] In an embodiment of the invention, for each region where
multiple masks are needed to create a continuous wafer shape, a
single additional edge segment is created on the inner tolerance
band layers for each mask taking part in creating the shape. The
location of the new inner tolerance band edge segment is somewhat
arbitrary within one critical constraint: it must allow a
continuous line to be drawn between the two tolerance bands
touching each side in order for the line to fall within two
adjacent tolerance bands and fall toward the "outer" side of the
new edge segment. Various methods can be used to create this
segment. However, it is preferable to directly connect the two
vertices on the inner tolerance band that are opened by
decomposition process. This is illustrated in FIG. 11(c) showing
vertex 1116 separating target edge segments 1115 and 1117,
pinpointing where the target is broken into two mask shapes.
Corresponding vertex 1119 on the inner tolerance band determines
where the inner tolerance band broke, forcing edge segments 1114
and 1118 to be moved to a separate mask. A corresponding vertex
1112 on the opposite side of the vertical section exists where
another break of the inner tolerance band occurs. A new edge
segment 1120 can then be drawn between 1112 and 1119 and a copy
placed on the inner tolerance band levels for each mask. This
results in closed shapes 1121 and 1122 for each inner tolerance
band which, in turn, results in the inner tolerance band for each
mask coinciding with each other for the segment 1120. The foregoing
has important implications to the robustness of the solution.
Alternatives will be discussed hereinafter.
[0046] Once the new inner tolerance band segments have been
generated in which corresponding outer tolerance band edges must
also be created for each mask. To find the outer band in one mask,
all other masks must be considered. For sake of clarity, only two
masks will be described hereinafter, although one may readily
recognize that it can be extended to multiple masks. For the first
mask, the shapes on the second mask are to be considered in order
to determine how far the new inner tolerance band edge segment can
move in a direction perpendicular to its length and into the
interior of the inner tolerance band shape prior to reach an
opposing edge of the outer tolerance band.
[0047] Referring to FIG. 12, numeral 1203 shows a copy of the new
inner tolerance band edge 1120 that was placed on Mask B. The edge
thereof is projected toward the interior of the inner tolerance
band shape 1202 until it extends beyond the shape and abuts at an
opposite edge on the outer tolerance band 1201 to form a new line
segment 1204. A rectangle 1206 is then drawn from the newly created
inner tolerance band edge 1120 to edge 1204. Next, the two edges
1208 and 1209 of the rectangle that are perpendicular to the newly
created inner tolerance band edge 1120 are expanded until they
reach the outer tolerance band of the second mask, a distance
depicted by numeral 1207. The shape 1210 created is then moved onto
the outer tolerance band layer of the first mask. Merging 1210 with
existing outer tolerance band shape 1105 results in a closed shape
for the outer tolerance band 1211 which, in conjunction with closed
inner tolerance band 1122, makes up a complete tolerance band for
the horizontal mask component on Mask A.
[0048] Referring to FIG. 13, similar steps are taken to create a
vertical mask component on Mask B. Numeral 1303 shows a copy of the
new inner tolerance band edge 1120 that was placed on Mask A. The
edge is projected toward the interior of the inner tolerance band
shape 1302 until it projects beyond the shape and reaches an
opposite edge on the outer tolerance band 1301 to create edge 1304.
Rectangle 1306 is then drawn from the newly created inner tolerance
band 1303 edge to the edge 1304. Next, the two edges 1308 and 1309
of the rectangle perpendicular to the newly created inner tolerance
band edge 1303 are expanded until abutting at the outer tolerance
band of the second mask, a distance depicted by numeral 1307. The
shape created 1310 is then moved onto the outer tolerance band
layer of the first mask, in which case, 1310 cannot be directly
merged with the existing outer tolerance band shape 1108. Instead,
the "cut" vertices 1311 and 1312 of 1108 are moved until they reach
the edge of 1310, i.e. a distance 1313. The shape is then merged
with 1108, resulting in a closed shape for the outer tolerance band
1314, which along with the closed inner tolerance band 1121, makes
up a complete tolerance band for the vertical mask component on
Mask B.
[0049] Referring now to FIG. 14, the patterns shown are that result
from masks used to print patterns in photoresist. Contour 1401 is
printed by Mask A, and 1402 by Mask B. The region 1403 shows an
overlap between the shapes printed by each mask. The overlap region
insures continuity of the desired T-shape. Still referring to FIG.
14, a second set of contours 1404 and 1405 is illustrated resulting
from printing the two masks on the smaller side of their expected
tolerance. Note that the small overlap region 1406 may cause yield
concerns.
[0050] Referring to FIGS. 15-18, in one embodiment, the method
described is aimed at improving the worst-case overlap of both
exposures, which is particularly important for overlay concerns for
double-patterning where secondary processes, such as etch. However,
these are not well understood, characterized or modeled. By way of
example, if an etch process is used after the lithography printing
process to create the final wafer pattern, the etch process may
actually shrink the patterns relative to the lithographic printed
patterns. In this case, if mask targets are created to print
lithographic wafer targets without accounting for a shrinking etch
process, the worst case overlap may not suffice to allow for
continuity of all the shapes printed with multiple exposures after
the final etch shrink occurs.
[0051] Referring to FIG. 15, a derived edge 1102 overlaid on the
pre-decomposed layout and tolerance bands is illustrated. The edge
is split into two separate edges 1501 and 1502 which are offset in
opposite directions by distances 1503 and 1504. Distances 1503 and
1504 should be equivalent or larger than the mask to mask overlay
specifications, while remaining smaller than the orthogonal width
of inner tolerances shapes 1505 and 1506.
[0052] FIGS. 16 and 17 show the use of the new edges 1501 and 1502
to create new decomposed inner tolerance bands, as well as further
improvements limiting the extent of the outer tolerance bands. To
enable a proper prediction of the final electrical performance and
to simplify certain aspects of a process window OPC, it is often
desirable to limit the size of the tolerance bands. Thus, referring
to FIG. 16, the inner and outer tolerance shapes for Mask A are
modified relative to corresponding ones of the FIGS. 11 and 12. In
such an instance, the new rectangle 1606 formed using edges 1102
and 1502 is added to Mask A inner tolerance shape 1122 to form a
new inner tolerance shape 1611. Edge 1502 is projected away from
1102 to form new edge 1601 in a manner similar to the creation of
edge 1204 (FIG. 12). However, the projection distance is limited to
distance 1605 which ensures that edge 1601 falls beyond the Mask A
outer tolerance band by a distance 1604 greater than or equal to 0,
while still ensuring that 1605 is sufficiently small to not
introduce additional complexity into the process window OPC
algorithm. The exact value of the distance 1605 is determined
through optimization of the process window OPC algorithm. This is
outside the scope of the present invention. Once distance 1605 is
determined and edge 1601 is created, rectangle 1607 can be formed
and expanded to 1609 in a manner similar to rectangle 1210 (FIG.
12). Likewise, the outer tolerance band shape 1610 is created in a
manner analogous to 1211.
[0053] Referring to FIG. 17, the creation of inner tolerance band
shape 1714 and outer tolerance band shape 1713 for Mask B is
illustrated in a manner similar to FIGS. 11 and 13. Herein, the
rectangle 1705 formed between edges 1102 and 1501 is added to the
inner tolerance band 1121 to create the new inner tolerance band
shape 1714. Edge 1501 is also projected away from 1102 by limited
distance 1704 to create new edge 1701. The rectangle 1706 bound by
the edges is then expanded by second limited distances 1708 and
1710 that ensure that the new outer tolerance edges are at least
aligned or extend beyond the current Mask B outer tolerance edges,
resulting in distances 1709 and 1711 being greater than or equal to
zero. The new rectangle 1712 formed in this operation is then
merged with the existing the Mask B outer tolerance band shape 1108
in a manner analogous to shape 1314 in FIG. 13 to form the new Mask
B outer tolerance shape 1713.
[0054] Tolerance bands created using the methodology illustrated by
FIGS. 15-19 can then be used to create Masks A and B using process
window OPC techniques as previously described. Printed wafer
contours resulting from double exposure of the masks is illustrated
in FIG. 18. Contours 1801 and 1802 from each exposure show a good
overlap in region 1802 at nominal process conditions. Likewise,
contours 1804 and 1805 printed at the smaller end of their
tolerance continue to show strong overlap in the region 1806,
resulting in a significantly improvement over the poor overlap 1406
(FIG. 140, resulting in an improved process latitude under
conditions of overlay variation and etch process biases.
[0055] FIG. 19 shows a flow chart describing a basic flow to create
the tolerance bands on multiple masks previously described and used
in conjunction with FIGS. 14 through 18, the combination of which
illustrates a preferred embodiment of the present invention.
[0056] The basic flow that creates tolerance bands on multiple
masks can be achieved in the following manner:
[0057] Create tolerance bands, mapping all the tolerance band edges
onto corresponding target edges (Blocks 1901, 1902)
[0058] Decompose the target edges and copy the corresponding
tolerance band edges onto the same masks as the target edges
(Blocks 1903, 1904).
[0059] For each break in the target shape created during
decomposition, create a new inner tolerance band edge at the
corresponding vertices. The inner tolerance band edges should be
created on all masks affected by the break in target shape (Bock
1905).
[0060] Find corresponding inner tolerance band vertices and connect
with a new line segment (Block 1906). For each mask affected by the
break in the target shape, consider all of the other masks (Block
1907) affected by the break and project the new inner tolerance
band edge in the direction of the interior of the inner tolerance
until it reaches an opposite edge of the outer tolerance band
(Block 1908). Create rectangles based on the projections keeping
the overlapping region of all of the rectangles (Block 1909).
[0061] Move the orthogonal edges of the new rectangle out until
they about at the outer tolerance band of one of the other affected
masks. (Block 1910) (1210, FIG. 12).
[0062] Move the "cut" vertices of the outer tolerance band being
fixed until they reach the new shape created in step 4 (Block 1910)
(1207, FIG. 12).
[0063] Performed by way of the aforementioned process (see
description of FIGS. 11 and 12 for the inner tolerance bands and
description of FIG. 13 for the outer tolerance bands) the
decomposed tolerance bands can be advantageously used to create
mask shapes for multiple masks (Block 1911).
[0064] Improve the overlap at all the mask junctions (Block
1912).
[0065] The present invention can be realized in hardware, software,
or a combination of hardware and software. The present invention
can be realized in a centralized fashion in one computer system or
in a distributed fashion where different elements are spread across
several interconnected computer systems. Any kind of computer
system--or other apparatus adapted for carrying out the methods
described herein--is suitable. A combination of hardware and
software could be a general purpose computer system with a computer
program that, when being loaded and executed, controls the computer
system such that it carries out the methods described herein.
[0066] The present invention can also be embedded in a computer
program product, which comprises all the features enabling the
implementation of the methods described herein, and which--when
loaded in a computer system--is able to carry out the methods.
[0067] Computer program means or computer program in the present
context mean any expression, in any language, code or notation, of
a set of instructions intended to cause a system having an
information processing capability to perform a particular function
either directly or after conversion to another language, code or
notation and/or reproduction in a different material form.
[0068] While the invention has been described in accordance with
certain preferred embodiments thereof, those skilled in the art
will understand the many modifications and enhancements which can
be made thereto without departing from the true scope and spirit of
the invention, which is limited only by the claims appended
below.
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